Many different electrodes and electrode configurations have been disclosed for applying electrical currents to the heart in an attempt to produce the most efficacious therapy and the least deleterious shock-induced alteration of myocardial electrophysiologic function. These electrodes have been placed directly on the epicardium of the heart, within the chambers of the right atrium and ventricle, in the coronary sinus, in the venae proximal to the right heart and in the left lateral thoracic subcutis. Various physical and electrical combinations of these electrodes form the electrode configuration.
In the past, the electrode configuration, as described above, most frequently employed in patients has been the epicardial patch-to-patch configuration as disclosed in U.S. Pat. No. 4,291,707 to Heilman et al. This prior art patch is bulky and relatively inflexible; the electrode is backed with an insulating layer of silicone rubber that increases transthoracic defibrillation shock strength requirements as reported by B. B. Lerman and O. C. Deale in Circulation 1990; 81:1409-1414.
More recently, a non-thoracotomy electrode configuration combining endocardial catheter electrodes with a mesh patch located in the left axillary subcutis was disclosed in U.S. Pat. No. 4,662,337 to Heilman, et al.
Still further potential disadvantages of the catheter-patch configuration are related to the impedance of the current pathway and the non-uniformity of the shock-induced electric field. High impedance pathways require higher shock intensities to defibrillate.
Prior art deployable epicardial defibrillation electrodes such as those disclosed in U.S. Pat. No. 4,567,900 to Moore undergo shape conversion subsequent to electrode placement. However, this design is believed to be fraught with problems related to structural frailty and it does not address either the problem of nonuniform current density at the electrode perimeter or fixation to adjacent tissues.
Additional disadvantages of prior art electrodes include excessive size, insufficient surface area, inefficient conductive discharge surfaces, excessive stiffness, nonconformity to heart shape, fatigue fracture, complex geometries, and complicated and hazardous implantation schemes.
Discussion in U.S. Pat. No. 5,016,645 to Willams et al. and U.S. Pat. No. 4,827,932 to Ideker et al. states either explicitly or implicitly that nonconductive backing of the defibrillation electrodes is necessary or beneficial to prevent shunting.
According to the present invention an opposite effect is realized, that is, unbacked structures require lower shock strengths for defibrillation than similar structures backed with a nonconductive material.